In modern automobile engines, pistons are designed to be relatively quiet under engine operating conditions and are produced with minimum tolerances. Instead of casting or machining the piston to a larger diameter, however, one method for producing such a piston has been to coat a piston skirt surface with a polymer coating. The polymer coating, in addition to any lubricant and piston rings, must withstand the mechanical reciprocation, temperatures, chemical contamination, and pressures that occur during engine operation.
The polymer coating is typically applied to a machined piston surface. Polymers are chosen that facilitate a minimal friction between the piston skirt surface and a cylinder wall. In some instances, however, a coating process can be insufficient or a piston surface can contain an imperfection. As a result, a bubbling or a delamination of the polymer coating from the piston can occur while in engine operation. The delaminated polymer can become a contaminant to the engine by accumulating in the oil. This reduces the engine efficiency and can even lead to an engine seizure An optimum adhesion of the polymer coating to a skirt surface on the piston is therefore desirable.
Screening new polymers, piston materials, and piston designs is expensive and time consuming. Traditional tests have included, for example, a cross-hatch adhesion test (ASTM D-3359), a high temperature engine oil bath test, a high temperature water bath test, and a high pressure oil impingement test. In the oil bath test, pistons are immersed in oil at 325° F. for 96 hours and any loss in coating adhesion recorded. In the water bath test, a piston having a polymer coating is submerged in high temperature water for a test-defined period of time. An appropriate pressure sensitive tape is then applied to the polymer and a minimum adhesive strength measured to determine a delamination resistance of the polymer coating (ASTM D-1000, Method A). In the oil impingement test, a high temperature oil jet is directed at the surface of the piston having a polymer coating. As with the water bath test, a pressure sensitive tape is then applied to the polymer coating to test for any delamination.
High frequency reciprocating tribology rigs (HFRR) for assessing the durability of moving parts, such as pistons, are also known in the art. Such test methods and equipment measure lubricity, or an ability of a fluid to affect friction between two surfaces. Wear to surfaces in relative motion under a load is also measured. Equipment used to perform such tests is manufactured, for example, by Imperial Scientific Industries.
An additional piston test has been reported by Slone in U.S. Pat. No. 5,007,284 and relates to a wear simulator for piston rings and cylinder liners. The wear simulator develops data on friction forces, wear, and friction coefficients based upon different speeds, loads, materials, temperatures, and lubricants.
There is a continuing need for a testing apparatus and method that more accurately represents the chemical and mechanical strain on the polymer skirt coating of the piston during engine operation. Desirably, the apparatus and method provide a more accurate prediction of piston delamination resistance.